The invention relates to a method of producing a positive or a negative object detection signal in dependence on the presence or non-presence of an object in a foreground region or a background region of a monitored region of an optoelectronic sensor, wherein during an operating phase transmitted light is transmitted in the direction of the monitored region, a plurality of received signals are produced in dependence on the reflected and on the received transmitted light, the received signals being associated with the foreground region or the background region, and a difference of the received signals is compared with a differential threshold to produce the positive or negative object detection signal, with a switching hysteresis being formed for a change between the positive and the negative object detection signal. The invention further relates to a corresponding optoelectronic sensor.
A detection should be made by such a method or by means of a corresponding sensor as to whether and when an object enters or leaves a specific part region of the monitored region, whereas objects in other spatial regions should be ignored. If, for example in the case of so-called background masking, an object changes from the foreground region into the background region, a switch from a positive to a negative object detection signal should be made.
In this connection, work is normally carried out according to the triangulation principle: a change in the distance of a reflecting object results in a displacement of the corresponding light spot on the receiver of the sensor, for example a differential photodiode. This displacement effects a corresponding change of the received signals. If the difference of the received signals hereby exceeds or falls below a differential threshold, a switch is made from a positive to a negative object detection signal, or vice versa. The differential threshold thus corresponds to a switch gap—with respect to the monitored region.
A switching hysteresis is provided to avoid an unwanted multiple switching between the positive object detection signal and the negative object detection signal with objects which are disposed or move in the switch gap. This is usually realized in that two different thresholds are used for the difference comparison, with the differential threshold used as the basis for the further difference comparison also being changed after a change in the object detection signal. This has the effect that the differential signal must now carry out a sufficiently large change in the opposite direction to again be able to cause a change in the object detection signal. The switching hysteresis therefore corresponds to a path hysteresis inside the monitored region.
The following problem can result here for some applications: to ensure an unambiguous detection of objects with irregular, reflecting surfaces, the switching hysteresis can admittedly be enlarged. However, since objects with different reflectance result in different differential signals for the respectively same scanning distance, the resulting path hysteresis is dependent on the reflectance of the object in question. Due to the differently steep profile of the differential signal, this results in dark objects having a larger path hysteresis than brighter objects; however, this is often unwanted.